1. Field of the Invention
The present invention relates generally to devices for monitoring the performance of optical systems. The present invention relates more particularly to devices for monitoring the performance of laser light sources used in communications and computation systems.
2. Related Art
The vertical cavity surface-emitting laser (VCSEL) is a relatively recent innovation in laser technology. It is part of a general class of devices called “surface emitting light emitting devices” (SLEDs) that have significant manufacturing and packaging advantages over conventional edge-emitting devices.
Semiconductor diode lasers have been produced for over a decade and are used extensively in both communications and in optical storage devices such as compact disks (CDs) and digital versatile disks (DVDs). The vast majority of these devices, however, rely on edge-emitting, e.g., Fabry-Perot or distributed feedback (DFB), lasers. These lasers are constructed on semiconductor wafers in such a way that when the wafer is diced, light is emitted from the cut edges. Edge emitting devices have a number of drawbacks: first, each laser takes a relatively large area on the semiconductor wafer, increasing cost; second, lasers cannot be tested until after they have been diced into individual units; third, linear arrays of lasers are more difficult to produce in high densities and two-dimensional arrays are altogether impossible to fabricate. The construction and fabrication of these lasers, however, is well known, and prices have benefited from large production volumes needed to satisfy the CD and DVD markets.
VCSEL laser cavities—rather than being patterned in the wafer plane, in a few layers of semiconductor—are patterned orthogonally to the wafer as many layers of semiconductor are deposited. The resulting lasers emit light perpendicularly to the surface of the wafer, and may be patterned in extremely high densities, either as individual devices or as one or two dimensional arrays. The result is a laser device that is inherently less expensive to produce than edge-emitting lasers. In addition, the vertical nature of these devices permits integration of additional electro-optical devices on the surface, for example adjacent each VCSEL.
Semiconductor light sources in general suffer from a number of problems associated with optical power control. Each semiconductor laser has a threshold electrical current needed before population inversion occurs i.e. there are more electrons in high energy state than in a low energy state, in its active region and coherent light is emitted. This threshold current needs to be supplied before any appreciable output is seen from a semiconductor laser. Above the threshold current, any increment in electrical current will lead to a corresponding increase in emitted optical power, up to a point. The ratio of the increase in optical power to electrical current is called the slope efficiency. Semiconductor lasers suffer from the fact that their threshold currents—and sometimes their slope efficiencies—can change significantly over operating temperature ranges and with laser age. This is a problem for a number of reasons. First, a single operating current cannot be set for the lifetime of the laser, unless it is set sufficiently high that output power will always exceed a desired minimum. This strategy has drawbacks: first, there may be eye safety issues when a laser is operated at greater than a certain power; second, operating the laser continuously at high power significantly reduces lifetime and further raises temperature, and as a result, threshold current; and finally, at high powers, high and low light output levels may be difficult to distinguish. Distinguishing between high and low light output levels is important in optical communications: a “one” and “zero” signal must be distinguishable to the receiver, and at the same time, the current levels for these signals should be as close together as possible in order to minimize switching time. It is therefore generally desirable to operate the laser at just above threshold for a “zero” level, and to use the minimum modulating current necessary to create “one” bits. The continuous current supplied is called the bias current. Thus a drift in the threshold current during operation can have highly detrimental results for users who wish to attain maximum bandwidth from such lasers while meeting eye safety and power consumption specifications. These optical power fluctuations are a problem not only for laser diodes or VCSELS; they affect other SLEDs as well, necessitating power control or monitoring for level-sensitive applications.
Various solutions have been developed for controlling optical output of diode lasers. The first category of solutions has to do with temperature monitoring and control. The idea is that one can either monitor temperature—or control it directly—of the laser device, and therefore eliminate drift in threshold current and slope efficiency tied to temperature fluctuations. The simplest solution is to place a temperature-monitoring device near the laser and to use the signal from this device to adjust the laser bias current and possibly the laser modulating current according to a pre-set formula determined from statistical sampling of the laser devices.
Another solution, which is used extensively in high-end communications modules employing edge-emitting lasers, is active control of laser temperature. The laser is placed on a substrate that has incorporated both a temperature-measuring device and a cooling device—most often a semiconductor heat pump such as a Peltier junction—that, through a control loop, keep the laser base at a constant temperature where the threshold current and slope efficiency are known (and usually optimal).
Thermal control solutions require significant space and power, and although they may be suitable for long-haul communications applications, such solutions are generally unacceptable in local-area or interconnect components where space is at an extreme premium.
Thermally-based solutions do not by themselves solve the problem of laser performance degradation over its operating lifetime. They can only compensate for changes in the ambient temperature, which, although important, are far from the only factor affecting laser optical output for a given current.
The most accurate way of controlling power output from the laser is to monitor the optical output directly. A class of technologies has been developed to monitor this output for both edge- and vertically-emitting semiconductor lasers.
Direct optical power monitoring for edge-emitting lasers is relatively straightforward due to the fact that these diode lasers emit light from both front and back facets. This allows the laser to be placed in an assembly where one aperture, at the front facet, provides the useful light for the application, while the other aperture provides light to a photodiode that is aligned precisely with the back facet. The usual technology used for this alignment is referred to as a silicon workbench. A silicon wafer has a surface patterned with mechanical alignment grooves using micromachining processes to produce a silicon workbench. Generally both the laser diode and the photodiode are placed in a “vee-groove” that runs along the light emission axis.
This type of assembly is used in CD and DVD players and recorders. For VCSELs, power monitoring is more complex, because the device does not generally emit light in the rear direction, i.e., through the substrate wafer. For laser wavelengths in excess of roughly 900 nm, a GaAs wafer, the usual VCSEL substrate would be transparent to the laser light. Thus, for such devices, an optical power monitor could be built on the reverse side of the wafer. However, VCSELS used for current communications applications generally operate in the 850 nm region for multimode fiber communications, and therefore generally require a different solution. The solution currently used by most manufacturers is to place the completed VCSEL die in an enclosure fitted with a partially reflective window above the VCSEL aperture and a photodiode onto which the partially reflective window projects some of the light from the VCSEL. Such an arrangement is called a backrefection monitor. Other solutions that have been proposed include photodiodes integrally built in the VCSEL structure using materials from columns III-V of the periodic table of the elements underneath the active layer; and photodiodes fabricated to monitor emissions from the side of the VCSEL structure. Both of these potential structures have not been used in production devices as a result of the significantly higher manufacturing complexities involved, and because of the fact that they do not directly monitor the same emission modes propagated by the surface-emitting device through its aperture, for example into a communication medium.
Current monitoring do not account for the fact that all lasers, even when fabricated closely together on a common substrate, may have very different characteristics. Many proposed applications, particularly in optical communications, require arrays of VCSELs used in parallel. Examples include high-speed interconnects built for server, router, or computer backplanes that feed into different waveguides. Although most arrays operate all devices at one wavelength, one proposed array would use an N×1 array of VCSEL, each tuned to a different wavelength combined with a multiplexer to provide wave-division multiplexed (WDM) communications capability. Such an array would have even greater power control issues than a single-wavelength array, because processing is slightly different for each laser. Conventionally, an additional VCSEL is constructed, at the end of array for the sole purpose of monitoring power output. However, this technique has severe drawbacks. Not only will it result in different average power levels coming from the lasers and, indeed, a higher required overall bias current than necessary in order to insure reliable function over lifetime, but may result in such disparities in power that an eye safety hazard results. This is of particular concern with a WDM system where multiple signals will travel through a single fiber to their destination, and aggregate optical power is measured to determine safety standards.
Another major drawback of current backreflection power monitors is the fact that light emanating from the VCSEL is not uniformly measured by the detector. Each VCSEL emits light not in a single beam or direction, but in various intensities in different directions off-axis, typically in a circular pattern. Typically VCSELs have beam divergence of 5-20°, with optical power unevenly distributed both by angle and radius in the beam. This poses a problem for an optical monitor that reflects part of the emitted light into a photodiode. This means that only a few of the modes of a VCSEL are measured by the photodiode. From VCSEL to VCSEL, then, the photocurrent produced in the detector will be vastly different, even for identical VCSEL output powers. The impractical result is that each VCSEL/monitor unit must be individually calibrated after assembly in order to know the relation between photodiode current and actual optical power produced by the VCSEL. This uncertainty is reflected in current product data sheets by the “photocurrent at typical VCSEL power,” which varies from min to max by as much as a factor of 10. Moreover, the relative intensities of the modes emitted by a VCSEL will change over temperature and age which means that the conditions observed during calibration may not exist over the entire operating lifetime of the VCSEL, and significant distortions of the power feedback signal may result over time. Other issues such as the mechanical and optical stability of the semireflective window used to direct light back to the photodetector will affect measurement as well. For example, dust on the outside of the window may cause significantly higher backreflection, leading to an overestimate of delivered laser power.
Current VCSEL/monitor units must exceed a certain minimum size because the partially-reflective window used to direct some light back towards the photodetector must be a set distance away in order to take advantage of the VCSEL's natural beam divergence. One partial solution to the problem has been to angle the window, which increases the reflected light and reduced the minimum distance required between the VCSEL and the window. However, even with this partial solution, direct integration of the VCSEL with waveguides—or, for that matter, incorporation into any other package that does not easily incorporate a window—is still impossible (assuming a power monitor is required). One of the VCSEL's potential strengths is that it could be directly coupled to waveguides and fibers, and bonded to a variety of surfaces; this strength of the VCSEL cannot be exploited using conventional technologies.
A highly desirable solution, unrealizable using conventional technologies, would be a semitransparent photodiode for monitoring laser diode output. All modes of the laser diode could be captured reliably achieving a consistent ratio between emitted optical power and photodiode current, and eliminating the calibration step during manufacturing. The overall VCSEL package could be made much more compact than at present if the photodiode could be added as a “layer” over the VCSEL. However, no such device has yet appeared because of numerous problems heretofore thought insurmountable.
One area of difficulty involves engineering a semiconductor device that has good responsivity to an optical signal of a desired wavelength; has a low dark current in order to provide sufficient contrast; transmits the majority of the light shining through it without excessively scattering it; has stable performance over a range of temperatures; is reliable over a long operating lifetime; and can be produced with consistent parameters. Even to experts in the field, the fabrication of devices meeting all these parameters has proven impossible.
Dark current in photodetectors is a perennial issue, particularly when the photodetector is stretched to meet other specifications such as responsivity at a particular wavelength and partial transparency. Dark current consists of two major components: that resulting from bulk semiconductor material and device properties, and that resulting from the specific construction of the device. The former component is minimized by carefully engineering the semiconductor layers in order to maximize the resulting signal-to-noise ratio. The latter component, that dependent on the specific construction of a single device, must be mitigated using fabrication methods. In the case of the semitransparent photodetector for laser power monitoring, a new and novel structure is required in order to minimize dark current while preserving functionality in the application-specific device. A particular problem is the leakage on the edges of the device.
In many cases it is desirable to build tall microelectronic structures for sensor or actuator applications. For many of these applications a top conductive contact with a lineout to a contact pad is required. When the conductor used for this connection is relatively thin when compared to the structure's height and is deposited by a directional method that preferentially deposits on surfaces parallel to the substrate broken connections often result.
Using standard metal deposition techniques like electron-beam deposition or thermal evaporation, metal is deposited perpendicular to the substrate, and therefore preferentially coats surfaces parallel to the plane of the substrate. An immediate potential solution was to use very thick layers of metal to form the contact once the layer is thick enough it will reach over the edge of the PIN stack; this fix, however, brings with it other problems when working with thin films. In particular, the film stress induced by such a thick layer will tend to peel off the entire photodetector structure.
In many cases it is desirable to build “tall” microelectronic structures for sensor or actuator applications. For many of these applications a top conductive contact with a lineout to a contact pad is required. When the conductor used for this connection is relatively thin when compared to the structure's height, and is deposited by a directional method that preferentially deposits on surfaces parallel to the substrate, broken connections often result.
Another problem is because the entire PIN structure was extended for large areas under the insulating layer and contacts i.e., areas that do not contribute to the intended photoresponse of the device, the dark current exhibited by the devices rose significantly for two reasons: (1) there is a much larger bulk of PIN stack that will produce purely thermal currents, particularly at higher temperatures and (2) the leakage currents produced along the extended edges of the structure rise. These effects are particularly noticeable when a high reverse bias is used to increase the response of the detector to light.